This project includes an investigaion of the uterine and placental factors which have effects on luteal maintenance in the rabbit. The investigators have demonstrated previously that the materanl placenta (decidua) in the rabbit can be produced by destruction of blastocysts hsortly after their attachment to the endometrium. This "blastocyst-induced" decidual tissue has a limited luteotrophic influence similar to that obtained by hysterectomizing early pregnant or pseudopregnant or pseudopregnant females. We are currently attempting to define the luteolytic effect of the non- pregnant uterus. Endometrial extractswith luteolytic properties have been obtained, but are not fully characterized rabbits and series of uterine giant cells to determine their possible roles in opposing the luteolytic effect of the uterus.